/Engine/Source/Runtime/GeometryCore/Public/Util/RefCountVector.h
Allocate at specificIndex, which must either be larger than current maxIndex, or on the free list. If larger, all elements up to this one will be pushed onto free list. otherwise we have to do a linear search through free list. If you are doing many of these, it is likely faster to useAllocateAtUnsafe(), and thenRebuildFreeList()after you are done.
bool AllocateAt ( int Index )